1. Field of the Invention
The present invention relates generally to a hermetically sealed waterproof casing for covering a drive unit which is supplied with electric power from a battery or a generator to drive an electrically-operated compressor mounted in an automotive vehicle and, more particularly, to a ventilation arrangement for such a casing.
2. Description of Related Art
An apparatus of a waterproof structure is generally provided with a hermetically sealed casing. However, in applications where the casing accommodates a heating element such as, for example, an electric component or the like, heat generated from the heating element causes expansion of air inside the casing, which in turn causes leakage of the air from between incompletely sealed portions of the casing. If operation of the heating element is consequently stopped, the air inside the casing contracts and, hence, air outside the casing is conversely sucked thereinto through the incompletely sealed portions. It is hardly possible to hermetically seal such incompletely sealed portions, because they include power lead wires made up of a core wire and a covering material. If water is drawn in together with the air, an electric circuit occasionally breaks down.
FIG. 11 depicts a casing 1 for covering a bath control unit installed in a wet bath room. This casing 1 has a vent hole 1a defined therein and a filter 2 mounted thereon so as to cover the vent hole 1a. To prevent water from entering the bath control unit, the filter 2 has a selective permeability to allow air to pass therethrough but repel water.
Accordingly, entry and exit of the air, which would be caused by thermal expansion and contraction of the air, are carried out through the filter 2 and not through the incompletely sealed portions, thus preventing entry of water.
A drive unit for driving an electrically-operated compressor for use in an automotive vehicle is required to be of a waterproof structure. It is therefore conceivable to form a vent hole in a casing of the drive unit and to cover the vent hole with an air-permeable and water-repellent filter as employed in the bath control unit.
However, unlike the inside of a house, automotive vehicles are exposed to a harsh environment, and attention must be paid to filter protection against water, salt, dust, oil, gases, mechanical stresses (impingement of an object against the filter) and the like.